FM radio signals can't transmit for longer distance than AM , Why?

Answer 1

Well, they could go the same distance, if there was nothing in the way.

In real life, there is stuff in the way. Things like mountains, hills, trees, concrete over your head in a building, etc. The reason that AM signals can penetrate those things better is because AM stations are all broadcasted on a longer "wavelength". Because the electromagnetic waves of AM stations *are* the size of hills and whatnot, they can therefore "go around" larger obstacles than can those of FM stations.

When you tune into a station, you do so by selecting its frequency. To find it's wavelength, you divide the speed of light (in meters per second or feet per second) by the frequency (in hertz).

So, the AM station at 1,000 kHz has a wavelength of about 300 meters.

The FM station at 100 mHz, on the other hand, has a much smaller wavelength: around 3 meters. Odds are, if you sat in a concrete bunker whose walls were thicker than 3 meters, you wouldn't be able to receive that station. You'd still pick up the 1,000 kHz AM station, though.

Answer 2

The frequency of AM radio is from 0.55 MHz up to 1.60 MHz. The frequency of FM radio is from 88 MHz up to 108 MHz. The two different frequency bands propagate over the Earth in different ways.

AM radio travels by 'ground wave'; the radio waves hug the ground and follow the curvature of the Earth over the horizon, so an AM receiver can hear a signal from an AM transmitter that is located relatively far away. In addtion, at night, the ionosphere is thicker and stronger, and this high altitude atmospheric layer acts like a mirror to radio waves in the AM band. So, AM stations can be heard over especially large distances at night.

FM signals, at the much higher frequencies of around 100 MHz, propagate only by "line of sight", which means if the transmitter is not visible directly by the receiver, the signal cannot be picked up. This also is the case for broadcast TV signals. That's why FM and TV transmitters tend to be put on hilltops or mountain peaks, or in flat lands, on tall towers or buildings. This gives greater line of sight distance for receivers for these signals.

So it has nothing to do with the modulation method (amplitude or frequency modulation). It has to do with the physics of radio propagation over the ground and through the sky.

Answer 3

AM will bounce and bend based on their relative frequency and the current atmospheric conditions. Since FM is at a higher frequency, the likelyhood of a reflection is minimal ... except when we have a sporatically active E-layer of the atmosphere that will reflect FM signals (and AM, for that matter) back towards the earth ... the whole angle of incidence = angle of refraction concept.

Tropspheric scatter will also effect transmission distance...once again, reflecting the radio waves to distance further than normal conditions.

Answer 4

think of FM signals as a little hyperactive child.... it goes reall well for a short time, but crashes hard after any kind of distance... AM signal is like a 30 year old marathon runner... it is not real powerful, but can go forever. the signal from FM is a very high frequency short wavelength signal... it dissipates in the atmoshpere because of dirt, and water.... (clouds) and also from other FM signals. AM is a low frequency long wavelength signal. and doesnt run into as much interference

Answer 5

AM radio waves bounce off an ionised layer in the upper atmosphere called the Heaviside layer. FM does not. This means that AM can bounce beyond the horizon, while FM cannot so that it can only reach receivers in front of the horizon.

The Heaviside layer is created by ionising radioation from the sun and so moves away from earth at night. This is why AM travels further at night.

Answer 6

FM waves are shorter so they scatter.
AM waves are much longer and during the daytime they tend to bounce off the ground for hundreds of metres. At night time, they can bounce off of the ionosphere which highly reflective (thank you sun!) and allow AM waves to go oh so very very far.

Because the FM waves are shorter, it means that they can carry higher quality of sound. AM is amplified modulation meaning that louder sounds amplified through a microphone take make longer waves. FM means frequency modulation with louder sounds causing shorter more frequent waves.

Answer 7

The frequency of AM lets it bounce of the ground, then the atmosphere, then the ground, then the atmosphere, so it goes a long way "over the horizon."

FM is a signal of a different frequency in which the frequency itself occillates, so it doesn't bounce as well and is a shorter-range signal.

Answer 8

AM is in KHz covers a lesser area easier to transmit but no stereo sound only mono(single channel)
FM is in MHz hence more power for the transmission of signal and cover a broader area sound can be transmitted both in Stereo(double channel better quality) or Mono(single channel travels further lesser quality).
haven't checked in years but you can find all the info at howstuffworks.com

Answer 9

AM has a wider bandwidth, FM has a shorter bandwidth. so basically.. this site will explain it.. hard for me to explain without drawing a picture :)

http://electronics.howstuffworks.com/radio-spectrum.htm

Answer 10

really? hmmm.... method of modulation has nothing to do with the distance...

it is the frequency and the power that make the difference.